This invention relates generally to liquefaction and evaporation of carbon dioxide and relates more particularly to a process and apparatus for liquefying a flow of relatively impure carbon dioxide gas in conjunction with providing a carbonation flow of relatively pure carbon dioxide gas.
Liquefied carbon dioxide has many applications, but is particularly useful in the beverage industry for carbonating beverages, such as beer and soft drinks. However, for the carbon dioxide to be of maximum usefulness, it must be relatively pure, i.e. free of contaminating gas such as oxygen and to a lesser extent nitrogen. If the carbon dioxide contains a significant amount of oxygen, the beverage in which it is used will be subject to oxidation and spoilage.
Various methods of liquefying gaseous carbon dioxide are well known. Typically, the liquefaction proccess comprises compressing the gaseous carbon dioxide to a pressure above atmospheric pressure and then removing the latent heat of vaporization to condense the compressed gas. In this way, although the sublimation temperature of solid carbon dioxide is approximately -109.degree. F. at STP, the compressed gaseous carbon dioxide can be condensed at much higher temperatures. The theoretical range of pressures over which gaseous carbon dioxide can be condensed to a liquid is approximately 60.45 to 1057.4 psig. Typically, such commercial processes processes operate in the range of approximately 225 to 300 psig. In this range, the temperature at which gaseous carbon dioxide will condense is -14.degree. F. at 225.25 psig and -8.degree. F. at 251.96 psig.
Since the carbon dioxide gas also contains contaminating gas with a lower temperature of condensation, and a different latent heat of vaporization, the contaminating gas requires a lower temperature and a different amount of heat removal to condense than does the carbon dioxide gas. Therefore, the carbon dioxide will condense before admixed contaminating gases, such as oxygen and nitrogen, will condense. The contaminating gas will therefore remain in a gaseous state, whereas the carbon dioxide will liquefy.
In typical liquefaction apparatus, gaseous carbon dioxide is passed through a tube which is surrounded by a refrigerant. The carbon dioxide condenses on the inside of the tube and collects in the bottom thereof. The temperature of the refrigerant must therefore be below the condensation temperature of carbon dioxide, but greater than the condensation temperature of the contaminating gas. The contaminating gas bubble is permitted to float to the top of the liquid carbon dioxide where it collects in a gas separation chamber.
In typical use, a supply of relatively pure liquid carbon dioxide obtained as above is vaporized and directed to a beverage carbonation process. Disadvantages result from the conventional approach of carrying out separately the liquefaction and evaporation processes in terms of the number of processing steps, energy requirements and the extent of apparatus required.